初探canvas制作一个折线图——（面向对象）

canvas

• 准备一块画布

 <style>
        canvas {
            border: 1px solid #ccc;
        }
    </style>
<canvas width="600" height="400"></canvas>

起步——start

<script>
    var myCanvas = document.querySelector('canvas');
    var ctx = myCanvas.getContext('2d');

    //构造函数
    var lineChart=function (ctx) {
        //获取绘图工具，上下文
       this.ctx=ctx||document.querySelector('canvas').getContext('2d');
       this.canvasWidth=this.ctx.canvas.width;
       this.canvasHeight=this.ctx.canvas.height;
       //网格大小
        this.gridSize=10;
        //坐标系的间距
        this.space=20;
        //箭头大小
        this.arrowSize=10;
        //原点
        this.x0=this.space;
        this.y0=this.canvasHeight-this.space;
        //绘制点
        this.dottedSize=6;
        //点的坐标——和数据有关系，数据可视化

    };
    //行为方法
    lineChart.prototype.init=function (data) {
        this.drawGrid();
        this.drawAxis();
        this.drawDotted(data);
    };
    //绘制网格
    lineChart.prototype.drawGrid=function () {
        //x的线
        var xLineTotal=Math.floor(this.canvasHeight/this.gridSize);
        for (var i=0;i<=xLineTotal;i++){
            this.ctx.beginPath();
            this.ctx.moveTo(0,i*this.gridSize-0.5);
            this.ctx.lineTo(this.canvasWidth,i*this.gridSize-0.5);
            this.ctx.strokeStyle='#eee';
            this.ctx.stroke();
        }
        //y方向的
        var yLineTotal=Math.floor(this.canvasWidth/this.gridSize);
        for (var i=0;i<=yLineTotal;i++){
            this.ctx.beginPath();
            this.ctx.moveTo(i*this.gridSize-0.5,0);
            this.ctx.lineTo(i*this.gridSize-0.5,this.canvasHeight);
            this.ctx.strokeStyle='#eee';
            this.ctx.stroke();

        }
    };
    //绘制坐标系
    lineChart.prototype.drawAxis=function () {
        //x轴
        this.ctx.beginPath();
        this.ctx.strokeStyle='#000';
        this.ctx.moveTo(this.x0,this.y0);
        this.ctx.lineTo(this.canvasWidth-this.space,this.y0);
        //画箭头
        this.ctx.lineTo(this.canvasWidth - this.space - this.arrowSize, this.y0 + this.arrowSize / 2);
        this.ctx.lineTo(this.canvasWidth - this.space - this.arrowSize, this.y0 - this.arrowSize / 2);
        this.ctx.lineTo(this.canvasWidth - this.space, this.y0);
        this.ctx.stroke();
        this.ctx.fill();
        //y轴
        this.ctx.beginPath();
        this.ctx.strokeStyle='#000';
        this.ctx.moveTo(this.x0,this.y0);
        this.ctx.lineTo(this.space,this.space);
        //箭头
        this.ctx.lineTo(this.space+this.arrowSize/2,this.space+this.arrowSize);
        this.ctx.lineTo(this.space-this.arrowSize/2,this.space+this.arrowSize);
        this.ctx.lineTo(this.space,this.space);
        this.ctx.stroke();
        this.ctx.fill();
    };
    //绘制所有点
    lineChart.prototype.drawDotted=function (data) {
        /*1.数据的坐标 需要转换 canvas坐标*/
        /*2.再进行点的绘制*/
        /*3.把线连起来*/
        var that = this;
        /*记录当前坐标*/
        var prevCanvasX = 0;
        var prevCanvasY = 0;
        data.forEach(function (item,i) {
            /* x = 原点的坐标 + 数据的坐标 */
            /* y = 原点的坐标 - 数据的坐标 */
            var canvasX=that.x0+item.x;
            var canvasY=that.y0-item.y;
            //绘制点
            that.ctx.beginPath();
            that.ctx.moveTo(canvasX - that.dottedSize / 2, canvasY - that.dottedSize / 2);
            that.ctx.lineTo(canvasX + that.dottedSize / 2, canvasY - that.dottedSize / 2);
            that.ctx.lineTo(canvasX + that.dottedSize / 2, canvasY + that.dottedSize / 2);
            that.ctx.lineTo(canvasX - that.dottedSize / 2, canvasY + that.dottedSize / 2);
            that.ctx.closePath();
            that.ctx.fill();
            /*点的连线*/
            /*当时第一个点的时候 起点是 x0 y0*/
            /*当时不是第一个点的时候 起点是 上一个点*/
            if(i == 0){
                that.ctx.beginPath();
                that.ctx.moveTo(that.x0,that.y0);
                that.ctx.lineTo(canvasX,canvasY);
                that.ctx.stroke();
            }else{
                /*上一个点*/
                that.ctx.beginPath();
                that.ctx.moveTo(prevCanvasX,prevCanvasY);
                that.ctx.lineTo(canvasX,canvasY);
                that.ctx.stroke();
            }
            /*记录当前的坐标，下一次要用*/
            prevCanvasX = canvasX;
            prevCanvasY = canvasY;
        });

    };
    //初始化
    这里的数据本应该是从后台传过来的json数据，这里模拟一下
    var data=[
        {x: 100, y: 120},
        {x: 200, y: 160},
        {x: 300, y: 240},
        {x: 400, y: 120},
        {x: 500, y: 80}
    ];


    var lineChart = new lineChart();
    lineChart.init(data);


</script>

效果